Loading on the components of wind turbines is carefully controlled, in order to maximise efficiency, and to minimise wear and damage. Load sensors are commonly mounted on wind turbines, typically on the rotor blades. These sensors are required to provide precise measurements of the loads on the turbine, and the sensors also require regular calibration in order to determine and track any offset between the values measured by the sensors, and the actual loads on the turbine components.
In one previously considered system for measuring loads on a wind turbine, there is a single sensor on each rotor blade. It measures the bending moment in only the “flap” or “flapwise” direction.
In another previous system there are four sensors that can measure bending moments in both the flapwise and edgewise directions. The sensors are mounted in the root of the blade and are aligned with the principle bending moment axes, “flap” and “edge”, as shown in FIG. 3, a cross section at the root of a rotor blade assembly. The rotor blade assembly (300) has an outboard blade section, indicated by dotted lines 304, and a blade root, having a circumference 302. The four sensors (306) are mounted on the circumference of the blade root. A first pair of the sensors are mounted in alignment with and at opposite ends of the edge bending moment axis (308), the axis through the front and rear edges of the blade. The second pair are mounted in alignment with and at opposite ends of the flap axis (310), orthogonal to the edge axis.
Having two sensors per axis allows differential measuring which is used to cancel out the axial forces (i.e. gravity and centrifugal) and provide a purer bending moment.
Blade designs have created problems with mounting the sensors along the principal axes (for example, structural shell blades with an internal web). There may also be other hardware, or the typical lightning conductor obstructing the alignment of the sensors. Previous implementations for calibration and operation of the blade load sensors have required that the sensors be placed along the principle bending moment axes, in order to provide accurate measurement and calibration. They have therefore incorporated inaccuracies, and are not sufficiently flexible regarding future development. They can also be expensive, due to the number of sensors required, and the complexity of installation and servicing.
Other previous systems have used pairs of sensors in different positions, such as one each on the edge and flap axes, or a pair aligned with one axis with a single sensor on the other. These nevertheless require the sensors to be aligned with the principle axes, or at least paired in parallel to an axis, as they are resolving forces separately in either the edgewise, or the flapwise direction.
The present invention aims to address these problems and provide improvements upon the known devices and methods.